Electron spin echo system having rf pulse inversion preparation of the spin echo sample

ABSTRACT

Method and apparatus for inverting the population of a pair of adjacent spin levels associated with a spin-echo sample when situated in a unidirectional magnetic field by means of a preparatory inversion RF pulse of a different RF frequency from the RF frequency of the input and recall RF pulses necessary for the generation of a spin-echo signal.

nited States Patent Eozanic et al.

[ 51 May 16, I972 ELECTRON SPIN ECHO SYSTEM HAVING RF PULSE INVERSIONPREPARATION OF THE SPIN ECHO SAMPLE Inventors: Donald A. Bozanic,Baltimore; Ronald W. Minarik, Lutherville; Dickron Mergerian, Baltimore,all of Md.

Westinghouse Electric Corporation, Pittsburgh, Pa.

Filed: June 30, 1970 App]. No.: 51,143

Assignee:

US. Cl ..324/0.5 R, 340/ I 73 Nl Int.Cl....... ..G0ln 27/78 Field ofSearch ..324/O.5; 340/ l 73 Nl DOUBLE PULSE SOURCEf 20 P. r I.

lnw. l usec I I2 PEEP. PULSE i SOURCE-fl l0mw.

[56] References Cited UNITED STATES PATENTS 2,714,714 8/1955 Anderson..324/0.5

OTHER PUBLICATIONS H. J. Gerritsen, S. E. Harrison & H. R. Lewis,Chromium- Doped Titania as a Maser Material, .lour. of App. Physics,31(9), Sept, 1960, PP. 1566- 1571 Primary ExaminerMichael J. LynchAttorney-F. H. Henson and E. P. Klipfel 57 ABSTRACT Method and apparatusfor inverting the population of a pair of adjacent spin levelsassociated with a spin-echo sample when situated in a unidirectionalmagnetic field by means of a preparatory inversion RF pulse of adifferent RF frequency from the RF frequency of the input and recall RFpulses necessary for the generation of a spin-echo signal.

12 Claims, 6 Drawing Figures Patented May 16, 1972 CIRCULATOR-I6 LDOUBLE PULS' mm SOURCE) 7 -IDETEET0R -IIN0I0AE0P| 1 LL 22 uoum HELIUM-26 4 I usec 2 24 PREP. PULSE 0c MAGNET- i souRcEn 32 lOmw. H0

4 F We RUTILE (Ti 0 SAMPLE30 FIG. 2-0

(PRIOR ART) T T F W T I.

. v v ECHO ST ND l PULSE 2 PULSE Z4 8K]- AT f AT f HG. 2b

ECHO PPN P P f ST ND 180 I 2 v INVERTING PREP. PULSE AT n LEVEL He 3POPULATION s\ 111 s 5 \/e 5 34 5 3 flw n2 n n HQ 4 LEVEL HG 3bPOPULATION ELECTRON SPIN ECHO SYSTEM HAVING RF PULSE INVERSIONPREPARATION OF THE SPIN ECHO SAMPLE CROSS REFERENCE TO RELATEDAPPLICATION The present application is related to an invention entitledElectron Spin Echo System Having a Pulsed Preparation Magnetic FieldApplied to the Sample, in U.S. Pat. No. 3,585,494, dated June 15, l97l,in the names of Donald Andrew Bozanic, et al. The assignee of thepresent invention is also the same as said referenced patentapplication.

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention isdirected to a method and apparatus for increasing the dynamic range of aspin-echo system wherein successive microwave pulses of a predeterminedRF frequency separated by a time interval T are fed into a microwavetransmission line in which a paramagnetic spin-echo sample is immersedin a fixed DC magnetic field thus establishing in said sample a state ofthermal equilibrium with a predetermined magnetization wherein theelectron spins have a population difference between a lower energy stateand a higher energy state. More particularly, the present invention isdirected to the concept of enhancing a spin-echo signal of a three ormore spin level system such as may be'obtained, for example, with apiece of titanium dioxide (rutile) as the spin-echo sample.

2. Description of the Prior Art In a spin-echo system a sample ofparamagnetic material is located in a resonant cavity situated within afixed strong uniform magnetic field and if the sample remains there fora sufficient period of time so as to be in thermal equilibrium i.e., theresultant magnetic moment present in the material will be aligned in thedirection of the field. When an RF field or inut pulse having afrequency equal to the characteristic or ,Larmor frequency of the sampleis applied at right angles to the fixed magnetic field, a torque isapplied to the moment which causes it to be tipped away from thedirection of the field. The angle of tipping, that is, the angle betweenthe moment and the direction of the field is proportional to the magnitude of the field and the time during which the RF field exists. Thetipping will be opposed by the magnetic field and since the spinningelectrons act like spinning gyroscopes their axes will precess about'anaxis parallel to the magnetic field in much the same manner as a tippedgyroscope. Subsequently, another RF field or recall" pulse which is alsodirected normal to the magnetic field is applied. After a quiescentperiod, the sample spontaneously develops a magnetic field of its ownwhich is also normal to the field and which rotates around the lattersdirection. The strength of this rotating field builds up and decayswhich is then detected as an electrical pulse called a spin echo"signal.

Pulsed nuclear induction spin echo methods and apparatus of the typementioned above are well known to those skilled in the art. An exampleof such a teaching is found in U.S. Pat. No. 2,887,673, issued to E. L.Hahn. The difference between electron and nuclear spin systems is thedifference in the operating frequencies encountered. This is attributedto the smaller mass of the electron in comparison to the proton in thenucleus. The precessional or Larmor frequencies of electrons lie in themicrowave range. Therefore, suitable microwave apparatus for applying atipping field at the same microwave frequency as the Larmor frequency ofthe sample is required in systems based upon electron spin echoes. Ingeneral, such a requirement is satisfied by the proper design andconstruction of microwave resonators in which the spin sample iscontained and in which RF pulses are applied and output spin echo pulsesobtained.

In stimulated spin echo systems such as disclosed in U.S. Pat. Nos.2,759,170, A. G. Anderson, et al. and U.S. Pat. No. 2,886,798, R. M.Walker, the object of these inventions is to provide storage of one ormore input pulses. A strong RF pulse termed the pre-pulse is applied toa system prior to the entries of weaker information" input pulses.Subsequent to the information entries, when it is desired to initiateecho fonnation, a second relatively heavy RF recall pulse is applied.This recall pulse is substantially identical with the pre-pulse andconverts the divergent moment relationships to convergent relationshipswith' the result that echo signals of the weaker information inputpulses appear. The echoes follow the recollection pulse in the sameorder and in the same time relationships as those by which theinformation input pulses followed the pre-pulse. The pre-pulse is ofsufiicient amplitude and duration to tip all the nuclear moments of thesample substantially through where during a predetermined time intervalthey are permitted to spread and distribute themselves throughout the XYplane by differential Larmor precession. Following this time interval,the information input pulses which are restored are applied. Thesepulses have the effect of depositing groups or families" of momentvectors on a system of cones revolving about the Z axis or direction ofthe field H The pulses are described as being entered into the Z axisstorage. The recollection pulse is of proper duration and amplitude totip the revolving moment cones again into the XY plane, at the same timehaving the effect of reversing the relative angular motions among theconstituents of each moment group. Thereupon the constituents of therespective groups reassemble to induce echo pulses in the system whichappear in the same order as their corresponding entry pulses.

Although the above noted spin-echo systems function as intended, thepresent invention is directed to a method and means for increasing thegain of the echo signal by changing the population difference of theelectrons contained in the atoms by pre-cooling the sample prior to theapplication of two RF microwave input pulses to the system forgenerating a spin echo signal.

SUMMARY Briefly, the inventive concept of the present invention isdirected to theimprovement in the output amplitude of a spin echo signalby applying a RF preparation pulse of a first microwave frequency priorto the application of the input (90) and recall (180) RF pulses both ofa same second microwave frequency to a spin echo system containing aparamagnetic sample having at least three spin levels which may be, forexample, iron doped titanium dioxide (rutile). A fixed DC magnetic fieldis applied whereupon the spin levels split, causing a populationdifference of the electronic spins to occur.

Normally the application of two successive microwave pulses at the samefrequency will cause an echo signal to be produced which is proportionalto the population difference between a predetermined first and a secondspin level. By applying an RF pre-pulse of a different frequency fromthat of the microwave pulses generating the spin echo, a 180 pulseinversion takes place between the second spin level and a third yethigher level, having the effect of taking all of the spins in the secondlevel and placing them in the third level, and vice versa. Since thenumber of spins in the second level has been reduced, the subsequentgeneration of a spin echo signal by the application of an input and arecall microwave pulse of the same frequency but different from thefrequency of the prepulse will have the relatively greater signal power.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of afirstembodiment of apparatus for practicing the subject invention;

FIGS. 2a and 2b are time related waveforms illustrative respectively ofthe operation of prior art apparatus and the embodiment shown in FIG. 1;and

FIGS. 30 and 3b are graphical representations of the Boltzmanndistribution of the population difference in the electron spins of aparamagnetic sample utilized by the subject invention and helpful inunderstanding the subject invention; and

FIG. 4 is a fragmentary perspective view of the location of theparamagnetic sample in the shorted end of a microwave transmission line.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawingsand particularly to FIG. 1, the block diagram illustrates a first sourceof microwave energy 10 having a frequency of operation 1i, and adaptedto produce microwave input and recall pulses P, and P, separated by apredetermined time interval T. The frequency F for example, lies in theX-band of microwave frequencies. The pulses P, and P, typically may be,for example, one microsecond in duration, and comprise about onemilliwatt of power. A second source of microwave energy 12 is disclosedhaving an operating frequencyf but in the same frequency band asj}. Itis adapted to provide a single preparation pulse output P having arelatively greater power content, for example milliwatts, as compared tothe pulses P, and P,. The triggering of both microwave sources 10 and 12may be accomplished either manually or by means of an electronicsynchronizer circuit, not shown. The P, and P, output from the doublepulse source 10 is coupled into a microwave transmission line 14 whichfeeds into a circulator 16. The second microwave source 12 couples itssingle pulse output P into a microwave transmission line 18 which isadapted to couple into the transmission line 14 by means of a microwavecoupler 20. One output port of the circulator 16 feeds into a microwavetransmission line comprising a waveguide section 22 which terminates ina cryostat 24 comprising a container filled with a cryogenic material 26which may be, for example, liquid helium. The end termination of themicrowave transmission line 22 includes a waveguide short 28thereacross. A paramagnetic spin echo sample 30 is located therein andis preferably comprised of a high dielectric material such as iron dopedtitanium dioxide, hereinafter referred to as rutile, having paramagneticdefect centers in the range of 10" to l0 defect centers per cm andhaving a dielectric constant in the order of 100. The rutile sample 30,being located in the waveguide transmission line 22 immersed in thecryogenic material 26, is adapted to operate at cryogenic temperatures,i.e. in the region of4 K.

A permanent magnet 32 produces a fixed DC magnetic field H, through therutile sample 30, causing the electron spins in the rutile sample tosplit into discrete energy levels that are populated according to theBoltzmann distribution so that i KT, CIC.

where n the number of electronic spins at a predetermined energy level,h Planks constant, f the Larmor operating frequency, K the Boltzmannconstant, and T= temperature in degrees Kelvin.

In prior art apparatus, a spin-echo signal at the frequency L, wasobtained by coupling the pair of f microwave pulses P, and P as shown inFIG. 2A into the rutile sample 30. It is well known to those skilled inthe art the first microwave pulse P, tips the axes of the electron spinsof the sample away from the direction of the magnetic field H causingthe spin orientation of the electrons to precess like an infinitesimalgyroscope. When the sample is subjected to the second, or P, pulse, thespins are flipped over 180. After a quiescent period equal in time tothe time interval T between the pulses P, and P, the sample develops amagnetic moment of its own normal to the field H, called the spin-echo"signal. This spin-echo signal is then coupled back into the wave guidetransmission line 22 through the circulator l6 and out another port intothe microwave transmission line 34 wherein the signal is detected bymeans of suitable apparatus 36 and observed by means of suitableindicator means 38 coupled to the detector means 36.

The power in the echo signal is proportional to the square of thepopulation difference between the two spin levels which are utilized.For example, referring to FIG. 30 at the operating frequency f,, thepower in the echo signal is proportional to the square of the differencebetween the number of electron spins n, in the lower, or state 1, andthe number of electronic spins n in the next higher state 2. Thus, P,,,,a(n,n,). The subject invention is directed to the concept ofincreasing the power in the echo signal by applying a preparatory pulseP, of a frequency f, to the rutile sample a predetermined time prior tothe application of the f,, microwave pulses P, and P,. The preparatorypulse P is of a selected power and duration to perform a 180 pulseinversion between the second energy level or state 2 as shown in FIG. 3band a selected higher level for example the third level or state 3,which has the effect of taking all of the spins n in the second leveland placing them in the third level while taking all of the spins 11;,in the third level and placing them in the second level. By doing thisbefore the generation of a spin echo signal, the the signal power of theof the subsequent echo signal produced by the pulses I, and P will thenbe P,,,,,,,a(n,n;,) Because n n there will be an increase in the echosignal power by the amount (n,- "a) "1 "2) For a Boltzmann population ofelectronic spin energy levels as shown in FIG. 3 and wherein thefrequencies f and f, are selected to be X-band microwave frequencies,the term hfJK- T l except at temperatures below 4 K. However, in the opeating range of 4 K an approximation can be made as follows:

h h 11 n, I andm, n,( so that,

n,n;, (m-mhfi/ V fr N E K (Al a ('11 -11) (n. m +lz /},/KT) a ft] Thusthe gain is proportional to the frequency ratio (LILY. It

is thus possible to obtain a gain of 20db by choosing typical X- bandfrequencies offl, 8 GHz andf, GH2.

By utilizing a spin echo sample comprised ofrutile selectively dopedwith iron group transition metal ions such as iron, manganese, cobalt,chromium and vanadium doped such that l0 to 10" defect centers per cmare produced the need for a resonant cavity is eliminated. Byeliminating the microwave cavity several advantages are gained. First,the restrictions on the homogeneity of the magnetic field H are reduced,since one utilizing a sample of rutile at X-band frequencies need onlyuse a very small sample in the order of 0.075 inches for properoperation. Secondly, the system is less bulky and simpler to manufacturesince one does not require a cavity with iris coupling holes, tuningarrangements, etc. Thirdly, the dielectric cavity modes observed atX-band frequencies in rutile couple very strongly to the spin system sothat a significant improvement in the insertion loss of the system overprior art apparatus utilizing for example quartz or silicon is obtained.Of the iron group transition metal ions utilized for doping, iron ispreferably as the paramagnetic impurity since the iron ion couples verystrongly to the spin-lattice.

Having thus described the present invention with respect to anillustrative embodiment thereof,

We claim as our invention:

1. An electron spin-echo system comprising in combination:

microwave transmission line means and a spin-echo sample locatedtherein;

DC magnet means located adjacent said microwave transmission line meansin the vicinity of said spin-echo sample to provide a fixedunidirectional magnetic field therethrough which splits the spins ofsaid sample into discrete energy levels having a Boltzmann distribution;

means generating and coupling a first pulse of RF energy at a firstpredetermined frequency to said microwave transmission line means andsaid spin-echo sample;

second means generating and coupling a pair of RF pulses of a secondfrequency different from said first predetermined frequency andseparated by a predetermined time interval to said microwavetransmission line means and said spin-echo sample subsequent to theapplication of said first RF pulse, said first RF pulse effecting a 180inversion to reverse the spin population between a selected pair ofelectron spin levels, whereupon said pair of RF pulses at said secondpredetermined frequency causes a spin-echo signal of said secondpredetermined frequency to be produced at a relatively higher powergain; and

means for coupling said spin-echo signal out of said transmission linemeans.

2. The invention as defined by claim 1 wherein said iron grouptransition metal ions are selected from the group consisting of iron,manganese, cobalt, chromium and vanadium.

3 The invention as defined by claim 1 wherein said spinecho sample iscomprised of iron doped rutile.

4. The invention as defined by claim 3 and additionally including meanssurrounding said transmission line means and said spin-echo sample forreducing the temperature of said sample to a predetermined cryogenictemperature.

5. The invention as defined by claim 4 wherein said means producing saidRF pulses include microwave signal generators.

6. The invention as defined by claim 5 wherein said microwave signalgenerators are comprised of X-band signal generators. I

LII

7. The invention as defined by claim 6 wherein said predeterminedcryogenic temperatures in the region of 4 Kelvrn.

8. The invention as defined in claim 1, wherein said sample comprisesrutile doped with iron group transition metal ions.

9. The invention as defined in claim 8, wherein said rutile sample has adoping concentration in the range of l X 10 to l X 10" defectcenters/cm.

10.The method of increasing the dynamic range of a spinecho signalcomprising the steps of:

generating a fixed DC magnetic field of a predetermined field strength;locating a spin-echo sample within a microwave transmission line andpositioning said transmission line containing said spin echo samplewithin said DC magnetic field;

applying a first pulse of RF energy of a first predetermined frequencyto said sample for inverting the spin populations of a selected pair ofelectron spin levels; and

subsequently applying a pair of pulses of RF energy of a secondpredetermined frequency different from the frequency of said first RFpulses in a predetermined time relationship for producing a spin-echosignal.

11. The method as defined by claim 10 and additionally including thestep of reducing the temperature of said spin-echo sample to apredetermined cryogenic temperature.

12. The method as defined by claim 11 wherein said sample comprisesrutile doped with iron group transition metal ions.

1. An electron spin-echo system comprising in combination: microwavetransmission line means and a spin-echo sample located therein; DCmagnet means located adjacent said microwave transmission line means inthe vicinity of said spin-echo sample to provide a fixed unidirectionalmagnetic field therethrough which splits the spins of said sample intodiscrete energy levels having a Boltzmann distribution; means generatingand coupling a first pulse of RF energy at a first predeterminedfrequency to said microwave transmission line means and said spin-echosample; second means generating and coupling a pair of RF pulses of asecond frequency different from said first predetermined frequency andseparated by a predetermined time interval to said microwavetransmission line means and said spin-echo sample subsequent to theapplication of said first RF pulse, said first RF pulse effecting a 180*inversion to reverse the spin population between a selected pair ofelectron spin levels, whereupon said pair of RF pulses at said secondpredetermined frequency causes a spin-echo signal of said secondpredetermined frequency to be produced at a relatively higher powergain; and means for coupling said spin-echo signal out of saidtransmission line means.
 2. The invention as defined by claim 1 whereinsaid iron group transition metal ions are selected from the groupconsisting of iron, manganese, cobalt, chromium and vanadium.
 3. Theinvention as defined by claim 1 wherein said spin-echo sample iscomprised of iron doped rutile.
 4. The invention as defined by claim 3and additionally including means surrounding said transmission linemeans and said spin-echo sample for reducing the temperature of saidsample to a predetermined cryogenic temperature.
 5. The invention asdefined by claim 4 wherein said means producing said RF pulses includemicrowave signal generators.
 6. The invention as defined by claim 5wherein said microwave signal generators are comprised of X-band signalgenerators.
 7. The invention as defined by claim 6 wherein saidpredetermined cryogenic temperatures in the region of 4* Kelvin.
 8. Theinvention as defined in claim 1, wherein said sample comprises rutiledoped with iron group transition metal ions.
 9. The invention as definedin claim 8, wherein said rutile sample has a doping concentration in therange of 1 X 1014 to 1 X 1018 defect centers/cm3.
 10. The method ofincreasing the dynamic range of a spin-echo signal comprising the stepsof: generating a fixed DC magnetic field of a predetermined fieldstrength; locating a spin-echo sample within a microwave transmissionline and positioning said transmission line containing said spin echosample within said DC magnetic field; applying a first pulse of RFenergy of a first predetermined frequency to said sample for invertingthe spin populations of a selected pair of electron spin levels; andsubsequently applying a pair of pulses of RF energy of a secondpredetermined frequency different from the frequency of said first RFpulses in a predetermined time relationship for producing a spin-echosignal.
 11. The method as defined by claim 10 and additionally includingthe step of reducing the temperature of said spin-echo sample to apredetermined cryogenic temperature.
 12. The method as defined by claim11 wherein said sample comprises rutile doped with iron group transitionmetal ions.